1. Field
The subject matter disclosed herein relates to obtaining position fixes using a position determination method in response to a change in a radio frequency (RF) environment.
2. Information
A satellite positioning system (SPS) typically comprises a system of transmitters positioned to enable entities to determine their location on the Earth based, at least in part, on signals received from the transmitters. Such a transmitter typically transmits a signal marked with a repeating pseudo-random noise (PN) code of a set number of chips and may be located on ground-based control stations, user equipment and/or space vehicles. In a particular example, such transmitters may be located on Earth orbiting satellites. For example, a satellite in a constellation of a Global Navigation Satellite System (GNSS) such as Global Positioning System (GPS), Galileo, Glonass, or Compass may transmit a signal marked with a PN code that is distinguishable from PN codes transmitted by other satellites in the constellation.
To estimate a position at a receiver, a navigation system may determine pseudorange measurements to satellites “in view” of the receiver using well known techniques based, at least in part, on detections of PN codes in signals received from the satellites. Such a pseudorange to a satellite may be determined based, at least in part, on a code phase detected in a received signal marked with a PN code associated with the satellite during a process of acquiring the received signal at a receiver. To acquire the received signal, a navigation system typically correlates the received signal with a locally generated PN code associated with a satellite. For example, such a navigation system typically correlates such a received signal with multiple code and/or time shifted versions of such a locally generated PN code. Detection of a particular time and/or code shifted version yielding a correlation result with the highest signal power may indicate a code phase associated with the acquired signal for use in measuring pseudorange as discussed above.
FIG. 1 illustrates an application of an SPS system, whereby a mobile station (MS) 100 in a wireless communications system may receive transmissions from satellites 102a, 102b, 102c, 102d in the line of sight to MS 100, and derives time measurements from a plurality of such transmissions. MS 100 may provide such measurements to location server 104, which may determine the position of the station from the measurements. Location server 104 may comprise any one of several platforms capable of determining the position of MS 100 including, for example, a position determination entity (PDE), serving mobile location center (SMLC), stand-alone SMLC (SAS) or a SUPL location platform (SUPL), just to name a few examples. Alternatively, MS 100 may determine its own position independently of such a location server.
MS 100 may search for a transmission from a particular satellite by correlating the PN code for the satellite with a received signal. The received signal typically comprises a composite of transmissions from one or more satellites within a line of sight to a receiver at MS 100 in the presence of noise. Though MS 100 may be capable of determining its position from received satellite transmission signals, buildings or other structures may “block” such signals, for example. In other words, whether or not a sufficient number of SPS satellites are in view of MS 100 may depend, at least in part, on the radio frequency (RF) environment of MS 100. In such an RF environment that may prevent MS 100 from acquiring signals from SPS satellites, determination of the position of MS 100 may instead be facilitated through communication with location server 104. Here, for example, MS 100 may obtain pseudorange measurements to terrestrial base stations using advanced forward link trilateration (AFLT) and forward such pseudorange measurements and other information (e.g., pseudorange measurements to one or more SPS satellites) to location server 104. Alternatively, MS 100 may employ other techniques for obtaining measurements for use in determining a range to a terrestrial base station such as, for example, observed time difference of arrival (OTDOA), enhanced observed time difference (E-OTD) and uplink time difference of arrival (U-TDOA), just to name a few examples.